Pentaerythritol purification process

ABSTRACT

Formate salts such as sodium formate are removed from a pentaerythritol solution in an electrodialysis unit divided by a stack of alternating cation and anion permselective membranes into an anode compartment, a series of alternating concentration and dilution compartments, and a cathode compartment. The anode compartment is contiguous to a concentration compartment and, preferably, the cathode compartment is contiguous to a dilution compartment. The impure pentaerythritol solution passes through the dilution compartments; an electrically conductive liquid, preferably a dilute solution of the impurity in the pentaerythritol solution, passes through the concentration compartments; and an electrolyte containing a salt, preferably one with the same cation as the formate salt in the pentaerythritol solution, is circulated from a common tank through the anode and cathode compartments and back to the tank.

[5 PENTAERYTIIRITOL PURIFICATION PROCESS Allyn H. Heit, San Mateo,Calif.

[73] Assignee: Sybron Corporation, Rochester,

[22] Filed: Nov. 3, 1972 [21] Appl. No.: 303,383

[75] Inventor:

[111 3,779,883 Dec, 18, 1973 [57] ABSTRACT Formate salts such as sodiumformate are removed from a pentaerythritol solution in anelectrodialysis unit divided by a stack of alternating cation and anionpermselective membranes into an anode compartment, a series ofalternating concentration and dilution compartments, and a cathodecompartment. The anode compartment is contiguous to a concentrationcompartment and, preferably, the cathode compartment is contiguous to adilution compartment. The impure pentaerythritol solution passes throughthe dilution compartments; an electrically conductive liquid, preferablya dilute solution of the impurity in the pentaerythritol solution,passes through the concentration compartments; and an electrolytecontaining a salt, preferably one with the same cation as the formatesalt in the pentaerythritol solution, is circulated from a common tankthrough the anode and cathode compartments and back to the tank.

12 Claims, 1 Drawing Figure [52] US. Cl 204/180 P, 204/72 [51] Int. ClB0ld 13/02 [58] Field of Search 204/180 P, 301, 72

[56] References Cited UNlTED STATES PATENTS 2,694,680 ll/l954 Katz etal. 204/180 P 2,865,822 l2/l958 Murphy 204/72 Primary Examiner-John H.Mackj Assistant Examiner-A. C; Prescott Attorney-Theodore B. RoesselC(CH Ot-IM CRYSTALS SEPARATOR CONCENTRATED NuCOOH SOLUTION DI LUTE NuCOOH SOLUTION C(CHgOHla SOLUTION ELECTROLYTE TANK EVAPORATOR C(CHzOH), 3

SOLUTION N0 30 NuOH PENTAERYTHRITOL PURIFICATION PROCESS BACKGROUND OFTHE INVENTION This invention relates to a process for purifying apentaerythritol solution, and more particularly to a process forremoving formate salts such as sodium formate or calcium formate from anaqueous solution of pentaerythritol by electro-dialysis Pentaerythritolis normally produced by reacting formaldehyde and acetaldehyde in thepresence of an alkaline catalyst such as sodium hydroxide or calciumhydroxide. The pentaerythritol is recovered in an aqueous solution thatalso contains formate salts such as sodium formate or calcium formate,depending on the alkaline catalyst used.

The pentaerythritol solution is usually purified in several evaporationand fractional crystallization steps. Large amounts of water arerequired to remove residual formate salts from the pentaerythritolcrystals. This process is relatively expensive because of themultiplicity of steps and the amounts of water required.

A process for purifying pentaerythritol solution by electrodialysis isdisclosed in U. S. Pat. No. 2,865,822 to C. R. Murphy. In'this process,a three or four compartment electrodialysis unit is used to produceformic acid and sodium hydroxide in addition to purifiedpentaerythritol. This process reduces the number of evaporation andfractional crystallization steps needed to purify the pentaerythritol,but is has several drawbacks. The process is limited to a three or fourcompartment stack because recovery of sodium hydroxide and formic acidare major goals. As a result, the process is relatively inefficient froman electrical standpoint because of the high voltage drop across theelectrode compartments. The voltage drop is particularly high in theanode compartment because of the oxidation of formic acid and the lowconductance of formic acid.

Another drawback of this process is the fact that sodium hydroxide andformic acid are corrosive. Thus, expensive materials must be used in thecell. In addition, formic acid is sensitive to the highly oxidizingconditions that exist in the anode compartment and is likely to breakdown into undesirable by-products. These drawbacks can be alleviatedsomewhat by using the four cell configuration shown in the patent.However, this configuration reduces the electrical efficiency of theprocess still further; and, since formic acid is only weakly ionized, itwill tend to diffuse into the anode compartment, accumulate there, andcreate the same .problems that the four cell arrangement was designed toeliminate,

SUMMARY OF THE INVENTION It is an object of this invention to provide anelectrodialysis process for the purification of pentaerythritol in whichthe voltage drop across the electrode compartments is minimized.

Another object of this invention is the provision of an is used as acommon electrolyte for both the anode and the cathode compartments.

A still further object is to provide an electrodialysis process forpurifying pentaerythritol in which spent anolyte is' reacted with spentcatholyte to produce fresh electrolyte.

This invention minimizes the formation of formic acid by using a dilutesalt solution as the anolyte in an electrodialysis unit with a cationpermselective membrane on the anode end of a stack of alternating cationand anion permselective membranes that divide the space between theanode compartment and the cathode compartment into alternatingconcentration and dilution compartments. Under the influence of electriccurrent, cations of the anolyte salt migrate through the cationpermselective membrane next to the anode compartment into the nextcompartment which, because of the membrane arrangement, is aconcentration compartment. In this compartment these cations unite withformate ions moving through the anion permselective membrane thatseparates this concentration compartment from the adjacent dilutioncompartment, which contains pentaerythritol solution to be purified.

The hydrogen ions generated at the anode combine with the free anions inthe anolyte. This, and the fact that formate ions are kept out of theanode compartment by the adjoining cation permselective membrane,minimize the generation of formic acid. This reduces the voltage drop inand the corrosiveness of the anolyte.

Since generation of formic acid is not a goal of this process, anelectrodialysis unit with a large number of dilution and concentrationcompartments may be used. This reduces the importance of the voltagedrop in the electrode compartments and makes the process more efficientelectrically. In addition, this minimizes the number of cells in whichcorrosive conditions tend to develop.

In the preferred embodiment of this invention, the electrodialysis unithas a cation permselective membrane on both ends of the membrane stack,and a common electrolyte containing a salt with the same cation as theformate salt in the pentaerythritol solution is used in both the anodeand the cathode compartments. Under these conditions, there is notransfer of anions from either the catholyte or the anolyte and cationsfrom the formate salt in the pentaerythritol solution are transferred tothe catholyte at the same rate at which cations are transferred from theanolyte. Thus, the total quantity of salt in the electrolyte remainsconstant and fresh electrolyte can be generated by mixing the spentcatholyte and the spent anolyte. Thus, there is no continualaccumulation of acidity or basicity. In addition, the regenerationeliminates the need for a supply of fresh electrolyte.

I Other objects and advantages of this invention will be apparent fromthe following description.

DRAWING DETAILED DESCRIPTION The pentaerythritol solution is purified inan electrodialysis unit l, with an anode 2 at one end and a cathode 3 atthe other end. The unit is divided by alternating cation pennselectivemembranes 4 and anion permselective membranes 5 into an anodecompartment 6, a cathode compartment 7, a concentration compartment 8and a dilution compartment 9. There is a cation permselective membraneat the anode end of the membrane stack, thus, there is a concentrationcompartment contiguous to anode chamber 6. In the preferred embodimentshown in the drawing there is is a cation permselective membrane on thecathode end of the stack as well. Thus, there is a dilution compartmentcontiguous to the cathode compartment 7.

Although the electrodialysis unit shown in the drawing has one dilutionand one concentration compartment, most units will have many more,because the effects of the electrode compartments are reduced inproportion to the number of compartments in the unit. A typical unitmight contain 200 or more membranes. The impure pentaerythritol solutionflows through inlet 10 into dilution compartment 9. A conductive liquid,which is preferably a dilute solution of the formate salt to be removedfrom the pentaerythritol solution, passes through inlet 11 intoconcentration chamber 8. Pump 12 forces electrolyte through lines 13 and14 to the anode compartment 6 and the cathode compartment 7. Theelectrolyte is a dilute conductive solution of a salt, preferably onewith the same cation as the impurity in the pentaerythritol solution. Inthe process shown, wherein the formate salt is sodium formate, saltssuch as sodium sulfate and disodium phosphate may be used in theelectrolyte. The preferred electrolyte is a 0.1 to 0.3 molar solution ofsodium sulfate, however, other salts which are not sensitive to theoxidizing and reducing condition in the anode and cathode compartmentsmay be used. Under the influence of electric current, sodium ions passfrom anode compartment 6 and dilution compartment 9 through the cationpermselective membranes 4 into concentration chamber 8 and cathodecompartment 7. Formate ions from the dilution compartment 9 migratethrough the anion permselective membrane 5 into concentrationcompartment 8. As a result, the sodium formate concentration in thesolution in dilution compartment 9 decreases and the soidum formateconcentration in the liquid in concentration compartment S-increases.

Hydrogen ions are generated at the anode, but they The sodium ions thatmigrate to the cathode compartment react with the hydroxyl ionsgenerated at the cathode to form sodium hydroxide. At the same time, thesulfate ions which are left behind in the anode compartment by themigration of sodium ions react with the hydrogen ions generated at theanode to form sulfuric acid. The rate at which sodium ions migrate fromthe anode compartment into the nearest concentration compartment matchesthe rate at which sodium ions migrate into the cathode compartment fromthe nearest dilution compartment. Since there is no migration of sulfateions from either the anode or the cathode compartments, the totalquantity of sodium and sulfate ions in the electrolyte remains constant,and stoichiometric amounts of sodium hydroxide and sulfuric acid areformed in the electrode compartments. The spent electrolyte is withdrawnfrom these compartments through lines 15 and 16 and mixed in tank 17,where the sodium hydroxide and sulfuric acid react to form sodiumsulfate, which is recirculated to the anode and cathode compartments.Since stoichiometric quantities of sodium hydroxide and sulfuric acidare formed in the electrode compartments, the quantity and concentrationof sodium sulfate in tank 17 may be maintained simply by adjusting theflow rates to and from the electrode compartments and adding water totank 17 to replace the water lost through electrolysis at the electrodesand the water transferred out of the anode compartment 6 with the sodiumions.

In order to avoid accumulation of gas and/or concentration polarizationin the electrolyte compartments, it is usually desirable to circulatethe electrolyte through these compartments at a relatively rapid rate.As a result, the anolyte does not become very acidic and the catholytedoes not become very basic. Thus, the anolyte and catholyte arerelatively non-corrosive and inexpensive materials can be used in theelectrode compartments.

The amount of purification which is achieved in a single stage unit ofthe type shown in the drawing may be controlled by adjusting currentdensity, the flow rate and/or the length of the path traveled by thesolution in passing through the dilution compartments. With typicalimpure solutions, which may contain 50 to 150 grams of pentaerythritolper liter and 40 to 130 grams 'of sodium formate per liter, up to 10percent of the sodium formate may be removed in a single pass. Sodiumformate concentrations as low as 0.3 gm/liter can be achieved by passingthe pentaerythritol solution through several electrodialysis stages ofthe type shown in the drawing, or in a batch operation, by recirculatingthe solution through a single unit.

After the desired purity is achieved, the pentaerythritol solutionpasses to one or more evaporators 19 wherein water vapor is driven offand one or more separators 20, which may be centrifuges, filters or thelike, where crystalline pentaerythritol is recovered. The evaporationand crystallization steps are much simpler than those employed inconventional pentaerythritol purification processes because most of thesodium formate has been removed by electrodialysis.

The concentration of sodium formate in concentration compartment 8 ismaintained at the desired level by adjusting the rate at which sodiumformate solution is withdrawn from the compartment. Typically, asolution containing up to to gms per liter of sodium formate iswithdrawn from the concentration compartment at a rate equal to thevolume increase of the concentrate liquor.

Although the foregoing description is of a continuous process, thisinvention may be practiced in batch processes as well. Many othermodifications may be made to the process described above, which ismerely illustrative and is not intended to limit the scope of thisinvention.

I claim:

1. In a process for removing a formate salt from a solution ofpentaerythritol in an electrodialysis unit with an anode compartment anda cathode compartment separated by a stack of alternating cation andanion permselective membranes, there being a cation permselectivemembrane at the anode end of the stack; the improvement wherein anelectrolyte comprising a dilute salt solution is circulated through theanode compartment.

- 2. A process according to claim 1 wherein the electrolyte salt has thesame cation as the formate salt.

3. A process according to claim 2 wherein the same dilute salt solutionis circulated through the anode compartment and the cathode compartment.

4. A process according to claim 3 wherein spent anolyte and spentcatholyte are mixed to form fresh electrolyte.

S. A process according to claim 4 wherein the formate salt is sodiumformate and the electrolyte is sodium sulfate or disodium phosphate.

6. A process according to claim 5 wherein the electrolyte salt is sodiumsulfate.

7. A process forremoving a formate salt from a pentaerythritol solutionin an electrodialysis unit with an anode compartment and a cathodecompartment separated by a stack of alternating cation and anionpermselective membranes that define alternating concentration anddilution compartments, with a cation permselective membrane at both endsof the stack, comprising:

a. placing the pentaerythritol solution in the dilution compartments;

b. placing an electrically conductive liquid in the concentrationcompartments;

c. placing a dilute electrolyte containing a salt with the same cationas the formate salt in said anode and cathode compartments, and

d. imposing an electric current across the unit, whereby anions of theformate salt pass through the anion permselective membranes into theconcentration compartment, cations of the formate salt pass through thecation permselective membranes into the concentration compartments andinto the cathode compartment, and cations of the salt in the electrolytepass from the anode compartment into the nearest concentration chamber.

8. A process according to claim 7 wherein the formate salt is sodiumformate.

9. A process according to claim 8 wherein the salt in the electrolyte issodium sulfate or disodium phos phate.

10. A process according to claim 8 wherein the salt in the electrolyteis sodium sulfate.

11. A process according to claim 7 wherein spent anolyte and spentcatholyte are mixed and recirculated to the anode and cathodecompartments.

12. A process according to claim 7 wherein the electrically conductivesolution in the concentration chambers is a dilute solution of theformate salt to be removed from the pentaerythritol solution.

UNETED STATES PATENT OFFICE CERTEFICATE OE CQRRECTKON Patent No. 3 I883Dated e ember 18, 1.973

Inventor(s) Allyn H611,

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Colnmn 1, line 8, change "electro-dialysiis" to "electrodialysisj.

Column 3, line 46, after "they" insert combine with the sulfate ionsfreed by the transfer of sodium ions out of the anolyte. Thnmdthere isessentially no hydrogen ion available to form formic acid inconcentration comoartment 8. There is even less chance of generatingformic acid in units with more than one concentration cell, because thecations entering additional concentration compartments come fromdilution compartments instead of from the anode compartment.

Column 5, line 8, after "electrolyte" insert salt Signed and sealed this23rd day of July 197 (SEAL) Attest:

M cor M. GIBSON JR. '0. MARSHALL DANN AEtesting Officer Commissioner ofPatents

2. A process according to claim 1 wherein the electrolyte salt has thesame cation as the formate salt.
 3. A process according to claim 2wherein the same dilute salt solution is circulated through the anodecompartment and the cathode compartment.
 4. A process according to claim3 wherein spent anolyte and spent catholyte are mixed to form freshelectrolyte.
 5. A process according to claim 4 wherein the formate saltis sodium formate and the electrolyte is sodium sulfate or disodiumphosphate.
 6. A process according to claim 5 wherein the electrolytesalt is sodium sulfate.
 7. A process for removing a formate salt from apentaerythritol solution in an electrodialysis unit with an anodecompartment and a cathode compartment separated by a stack ofalternating cation and anion permselective membranes that definealternating concentration and dilution compartments, with a cationpermselective membrane at both ends of the stack, comprising: a. placingthe pentaerythritol solution in the dilution compartments; b. placing anelectrically conductive liquid in the concentration compartments; c.placing a dilute electrolyte containing a salt with the same cation asthe formate salt in said anode and cathode compartments, and d. imposingan electric current across the unit, whereby anions of the formate saltpass through the anion permselective membranes into the concentrationcompartment, cations of the formate salt pass through the cationpermselective membranes into the concenTration compartments and into thecathode compartment, and cations of the salt in the electrolyte passfrom the anode compartment into the nearest concentration chamber.
 8. Aprocess according to claim 7 wherein the formate salt is sodium formate.9. A process according to claim 8 wherein the salt in the electrolyte issodium sulfate or disodium phosphate.
 10. A process according to claim 8wherein the salt in the electrolyte is sodium sulfate.
 11. A processaccording to claim 7 wherein spent anolyte and spent catholyte are mixedand recirculated to the anode and cathode compartments.
 12. A processaccording to claim 7 wherein the electrically conductive solution in theconcentration chambers is a dilute solution of the formate salt to beremoved from the pentaerythritol solution.